A cooking oven that has both a microwave mode and an impingement mode is shown in U.S. Pat. No. 5,254,823 as an oven that has a rather large preheated thermal reservoir (at least 60 pounds) so as to facilitate rapid heat transfer to ambient air in a plenum. However, such an oven is quite heavy and cumbersome for many applications. Moreover, the preheat time is considerable (up to two or more hours) and cooling of the oven's exterior surfaces can be difficult and energy inefficient.
There is a need for an oven that conserves energy during non-cooking time.